The present invention relates to a spatial light modulator wherein an electro-optic crystal is arranged against the electron beam source within a vacuum envelope so as to store electrons emitted from the electron beam source onto the surface of the crystal, to change the refractive index corresponding to the charge stored on the crystal surface, and to read out the change in the refractive index by means of a laser beam.
The fundamental operation of the spatial light modulator will be described hereinafter.
FIG. 1 shows a schematic diagram of the spatial light modulator wherein an input optical image 1 is formed on a photoelectric layer 4 inside a glass envelope 3 of the spatial light modulator after passing through a lens 2 during illumination.
Photoelectric layer 4 emits photoelectrons responding to the input image. The photoelectrons are incident on a microchannel plate 6, after passing through an accelerating and focusing lens 5, and are multiplied by a factor of the order of thousands. The multiplied electrons, after passing through a mesh electrode 7, are stored on one surface of an electro-optic crystal 8 whose other surface is covered with a transparent film electrode 8a, made for instance of LiNbO.sub.3, to change the refractive index of crystal 8 in response to the electric charge image.
When the laser beam from a laser beam source 10 is incident on crystal 8 after passing through a half mirror 9, a laser beam image 11 or a coherent image can be obtained. This image is used in many optical image processing and optical computing system.
If electro-optic crystal 8 of the spatial light modulator is relatively thick, the electric field caused by a point charge P on a surface of the electro-optic crystal 8 spreads out as shown by .delta..sub.1 in FIG. 2(A).
If electro-optic crystal 8 of the spatial light modulator is relatively thin, the spreading range .delta..sub.2 is much smaller than .delta..sub.1, as shown in FIG. 2(B).
This electric field changes the refractive index of crystal 8 and it modulates the magnitude of the phase of the light beam sent from laser beam source 10. It can easily be understood that the resulting coherent light image has high resolution for the thinner electro-optic crystal 8.
Electro-optical crystal 8 was sliced as thin as possible with a flatness of .lambda./10 or better while the surfaces thereof were kept parallel with a parallelism of five seconds or less. That is, a LiNbO.sub.3 single crystal plate was cut with the normal to its surface located in the (-Y,Z) plane and at an angle of 55 degrees with respect to the Z or optic axis of the crystal. The crystal, having a diameter of 25 mm, was sliced into a wafer with a thickness of 0.3 mm. A spatial light modulator of this type has a resolution of three line-pairs/mm (at a modulation factor of 50%).
The resolution of three line-pairs/mm, however, is unsatisfactory for optical image processing and optical computing, and a thinner electro-optic crystal 8 is required for higher resolution. Thinner LiNbO.sub.3 single crystal plates might mechanically be distorted to such an extent that they could not be used for the above objectives.
The objective of the present invention is to provide a spatial light modulator with high resolution utilizing a very thin electro-optic crystal plate.